Note that the downstream velocity and countercurrent velocity depend on the velocity in still water and the velocity of water flow.

Velocity in still water+current velocity = downstream velocity

Velocity in still water-current velocity = countercurrent velocity

Let the velocity in still water be x and the velocity of water flow be y.

x+y=300/ 15

x-y=300/20

solve

x= 17.5

y=2.5